Alternations are without a doubt my favorite training session. No session does more to directly impact your fitness. No session impacts more systems. No session leads to bigger jumps in fitness.
That said we have not always been such great friends my first attempt at these can only be described as a disaster. It was two weeks prior to my debut marathon. I was fit. I had run a big half marathon PB the weekend before and I wanted to get in one last hard marathon workout before starting my taper. I decided on trying out an alternation session. 20 kilometers of 1k at half marathon pace and 1k recovery at about 5% slower than half marathon pace. My goal was to average about 3:12 per Kilometer which is 2:15 marathon pace. I wanted to run in the 3:02 range for the 'on' or fast kilometers and around 3:20 to 3:22 for the 'off' or recovery k's. Now my regular training pace at the time was around 6:15 per mile and 3:20 per K is 5:20 mile pace so it is definitely not an easy pace but it was slower than the goal marathon pace I was targeting and I had at this point done runs around 18 miles at low 5:20 pace so it was a pace I could relax at. Or so I thought. I started with the fast k and for the first couple reps things seemed fine. Then I during the 2nd rest K I noticed I simply wasn't recovering. I was still breathing very heavy at the start of the 3rd fast K the next recovery K involved no recovering what so ever. I was running hard to hit the 3:20 and I was almost surprised I was able to accelerate at all entering my 7th overall kilometer, the 4th 'fast' K. I finished the 7th K and collapsed. I was completely finished. At the time this was super stressful. I was not super confident in my fitness. I had run a few workouts and one race massively better than I had ever run before but this was a workout a 2:15 to 2:16 marathoner should be able to do and in two weeks I had to try and run that. There was no time left to do anymore workouts all I could do was taper and hope.
Thing is the race arrived and I ran 2:15:28. My fitness was there! So why had I failed so miserably at the alternation session? The alternations have some very specific demands and you need to build up to doing it. I had not.
If I was still able to run fast so why bother with the alternations? For this another story an athlete I help out was prepping for his first marathon. He was very fit. He had done some workouts that showed in ideal conditions he should run 2:13 to 2:15. He had not done any alternation sessions. He went to Rotterdam. There wasn't a pace group at his pace. He opted to be aggressive and went out on 2:11 mid pace. Quick but not a pace that should have been suicidal. He was through half way just under 1:06. Reality started to sink in. Alternations would not have prevented this. He slowed to about goal pace, 16:00, for the 5k from 20k to 25k. Then he continued to slow from 25k to 30k he ran around 16:40, 5:20 mile pace. The pace a runner like him would learn to recover at by doing alternations. This is wear alternations would have helped hugely. My athlete did not recover. If he had he would have been able to run that same pace, 3:20 per K to the finish and he would have run around 2:15 or he may have even been able to pick up the pace closer to his goal pace and he would have run even faster. Instead he continued to struggle and faded to a 2:18. Now this was still a respectable debut he had been able to avoid a real disaster because he had done enough other specific work but still by not forcing alternations on him and hoping for perfect pacing. In short I failed him.
So now how to do marathon specific alternations. Your goal is to do around 20k of total running. If you are a real workout champ you may want to more like 25k. If you prefer imperial distances 12 to 15 miles. There is no exact distance for your reps. You can pick your distance really anywhere from 400m to 2 kilometers. What you do need to do is match the distance of the rep to the distance of the rest. Or at least that should be your goal at the end of the cycle. Ie you want to finish with 20 to 25k of say 1k at 5% faster than marathon pace efforts and 1k at 5% slower than marathon pace. Now as my first story illustrates just trying to jump into that is a recipe for failure.
Where to begin. You have a couple choices. Either way you want the total distance to be in the 20 to 25k range.
Choice 1 is to run equal distances for rest and rep but to start with a much slower pace on the rest. Instead of starting at 5% slower than marathon pace you want to start at more like 20% slower than marathon pace, which is probably about the fast end of your normal training pace. This will obviously lead to a slower overall time but it is the beginning it is fine to be slower. If you go this way each time you repeat the session you try and run the rest a little faster. This will also improve the overall time of your 20k towards that average of marathon pace you are looking for.
Choice 2 is to run shorter reps and longer rests but keep the paces at 5% above and below marathon pace respectively. So your starting point might be 400m at 5% faster than marathon pace with recoveries of 1600m at 5% slower than marathon pace. Like the above option this means your average pace will at the beginning be slower than marathon pace but like above that is fine. The point of a workout is to improve so if you can run it in its final form at the beginning you probably don't need that workout. Each time you repeat the workout you will go further at the faster pace and shorter at the slower pace. So for the second session you might go 500/1100. Building towards your goal of 1k/1k. You would of course adjust these numbers if your goal was to run 500/500 or 2k/2k at the end of the cycle.
When to do this session. In terms of the training cycle I would start this in the mid to early base and use it right through the specific phase with 1 to 3 sessions where you average marathon pace in the specific phase. If you are able to do 20k at marathon pace at the start of your specific phase you should increase the distance when you repeat the workout later in specific phase rather than trying to faster.
In terms of which cycles you want to do this. As I mentioned above if you think you may not be able to run a very even pace in your goal marathon this session is key. Also if you have struggled with the Wall in the past, running out of glycogen, this session can be key to improving your ability to burn more fat at marathon pace and avoiding that problem. Finally if you need to race a good half marathon in the weeks before your goal marathon this session is great because you get a lot of work at half marathon pace as well.
So there you have it marathon specific alternations. It is one of many types of alternations but this particular session is certainly one of my favorites.